1. Halogenoalkanes

2. Halogenoalkanes
substitution reactions of halogenoalkanes with hydroxide
Explain why the hydroxide ion is a better nucleophile than water
Describe and explain rates of nucleophilic substitution
Explain how the rate of Sn1/Sn2 of halogenoalkanes by OH- changes

3. Animation:

4. Nucleophilic Substitution RX on primary halogenoalkane: SN2 Mechanisms
Chemistry 140 Fall 2002
C2H5Cl + OH-  C2H5OH + Cl-
chloroethane ethanol
Chemical research, starting in the 1890s, has shown that nucleophilic substitution reactions can involve two types of mechanisms. The reaction between chloromethane and the hydroxide ion (27.2) has a rate law that is first order in both the nucleophile and the electrophile.

5. rate=k[OH-][CH3CH2Cl] proportional to both reactants= order 2
Bimolecular Reaction :
rate=k[OH-][CH3CH2Cl] proportional to both reactants= order 2
Inversion of the arrangement of the atoms around C = Stereospecific.
Faster reaction favored by aprotic and polar solvent:
OH- is not solvated by solvent and reacts faster.

6. Reaction profile for an SN2 reaction ( ONE STEP- one Ea )

7. rate =k[(CH3)3CCl] determined by unimolecular slow step = order 1
Nucleophilic Substitution RX on tertiary halogenoalkane:
SN1 Mechanisms
(CH3)3CCl + OH-  (CH3)3COH + Cl-
2-chloro-2-methylpropane methyl-propan-2-ol
Steric hindrance:
Difficult for incoming group to react due to bulky 3 CH3 groups
Unimolecular Rx
Positive induction:
Stabilization effect on the carbocation
rate =k[(CH3)3CCl] determined by unimolecular slow step = order 1

8. OH- Y Cl
Cl-
OH-
During both steps, there is some arrangement of the atoms which causes an energy maximum. This is called a transition state ( ts1 and ts2 )
The stability of the carbocation intermediate is shown by the fact that there are small activation barriers to its conversion either into product or back into reactants.

9. Reaction favored by protic and polar solvent (with OH or NH) able to
stabilize the intermediate carbocation by ion-dipole interaction and the
leaving group ( ex: Cl-) by proton.

10. Tertiary C > Secondary > Primary SN1 SN1- SN2 SN2
Comparison of the rates of nucleophilic substitutions SN1 SN2
Mechanism:
Tertiary C > Secondary > Primary
SN SN1- SN SN2
Leaving halogen:
C-I > C-Br > C-Cl > C-F
longer- weaker shorter-stronger
Solvent:
- SN1 favored by polar, protic solvent (with OH or NH). Stabilization of carbocation.
- SN2 favored by polar, aprotic solvent. OH- non solvated (bare)
The faster reactions (SN1) occur with tertiary iodoalkanes in polar, protic solvents.

11. rate = k [OH-] [CH3Cl] rate = k [CH3Cl]
Rate of reaction
lower ( C-I> C-Br > C-Cl > C-F) higher
rate = k [OH-] [CH3Cl]
rate = k [CH3Cl]

12. One more tutorial on :
-SN2
-SN1
-Solvent effect
One last animation :
We SN1 and SN2